Reliability performance of titanium sputter coated Ni–Ti arch wires: Mechanical performance and nickel release evaluation

Abstract

The present research was aimed at developing surface coatings on NiTi archwires capable of protection against nickel release and to investigate the stability, mechanical performance and prevention of nickel release of titanium sputter coated NiTi arch wires. Coated and uncoated specimens immersed in artificial saliva were subjected to critical evaluation of parameters such as surface analysis, mechanical testing, element release, friction coefficient and adhesion of the coating. Titanium coatings exhibited high reliability on exposure even for a prolonged period of 30 days in artificial saliva. The coatings were found to be relatively stable on linear scratch test with reduced frictional coefficient compared to uncoated samples. Titanium sputtering adhered well with the Ni–Ti substrates at the molecular level, this was further confirmed by Inductive coupled plasma emission spectroscopy (ICPE) analysis which showed no dissolution of nickel in the artificial saliva. Titanium sputter coatings seem to be promising for nickel sensitive patients. The study confirmed the superior nature of the coating, evident as reduced surface roughness, friction coefficient, good adhesion and minimal hardness and elastic modulus variations in artificial saliva over a given time period.

Keywords

NiTi alloy Ni allergy sputtering titanium nanoindentation

1. Introduction

The utilization of Ni–Ti (Nickel titanium) orthodontic arch wires in the leveling and alignment stages of treatment has increased significantly [1–5]. Despite the excellent properties of Ni–Ti wires (composition: nickel > 50 wt%), Ni ions released from the alloy can cause allergy in nickel sensitive patients [6–10].

Alternative materials for nickel allergy patients includes the usage of nickel free alloys like twist flex stainless steel, fibre-reinforced composite arch wires and titanium molybdenum arch wires or provide some stable nickel free coatings onto these Ni–Ti alloys [1,10]. However, polymer coatings like epoxy resin have the problem of adherence, while the ceramic coatings are more prone to brittleness [11]. Metals like Gold (Au), Platinum (Pt), Titanium (Ti) and/or their alloys could be used for coating. Amongst which titanium is the most biocompatible and shows equivalent corrosion resistance as a result of the passivating effect afforded by a thin layer of titanium oxide that is formed on its surface [12].

Different techniques available to coat titanium onto the Ni–Ti arch wires include surface oxidation heat treatment, chemical vapor deposition, physical vapor deposition and ion implantation methods [13]. In the surface oxidation, Ni–Ti wires were heated to 500°C which caused deterioration in the properties of the Ni–Ti alloy. In the chemical vapor deposition [14] films are deposited at elevated temperatures (200–1600°C). This puts some restrictions on the kind of substrates that can be coated. Several studies reported that [15–17] ion implantation process are incapable of decreasing the amount of nickel released into the oral environment or providing beneficial corrosion properties. Physical vapor deposition (PVD) coatings [18] in general help to improve hardness, wear resistance and oxidation resistance, at the same time reducing the frictional characteristics [19,20]. There are many variants of the PVD coating process, such as electron beam PVD, evaporative deposition, sputtering, cathodic arc deposition and pulsed laser deposition. Sputtering [21] is a widely used and highly versatile vacuum coating system which makes use of plasma for thin film deposition. The present study explores the possibility of utilizing thin film titanium sputter coatings onto Ni–Ti arch wires. These coated arch wires were subjected to artificial saliva environment and were tested for stability, mechanical properties and prevention of nickel release.

2. Materials and methods

2.1. Materials

NiTi arch wires with cross-sectional dimensions of ${0.017}^{″} \times {0.025}^{″}$ were obtained from 3M Unitek (Monrovia, CA). The as-received wires were cut into 25 mm segments with a slow-speed, water-cooled diamond saw.

2.2. Methods

2.2.1. Preparation of titanium sputter coating

High vacuum plasma ion titanium sputtering (EMITECH K575X) was carried out on Ni–Ti arch wires with a titanium disc (>99.9%) as the sputtering target. Prior to the coating, the samples were sonicated for 30 min in acetone. The distance between the target and the substrate was about 35 mm, and the diameter of the target was 55 mm. The sputtering chamber was evacuated to about 10⁻³ torr vacuum. Argon gas (99.999%) was purged into the chamber and the pressure was maintained to about 10⁻¹ torr. The coating procedure was carried out at a sputter current of 120 mA for 5 min. The samples were divided into 2 groups: (a) coated and (b) uncoated. Each group comprised of 20 samples.

2.2.2. Immersion test

Artificial saliva was prepared according to the formulation prescribed by Fusayama Meyer [22], whose composition comprised of KCl (0.4 g/l), NaCl (0.4 g/l), CaCl₂ · 2H₂O (0.906 g/l), NaH₂PO₄ · 2H₂O (0.690 g/l), Na₂S · 9H₂O (0.005 g/l) and urea (1 g/l). The pH was maintained in the range of 5.5–6.5. Coated and uncoated samples were immersed in 20 ml of artificial saliva for period of 0, 1, 10 and 30 days. The temperature was maintained in the range of 37 ± 1°C.

2.2.3. Surface characterization

Surface morphology of the coated and uncoated wires were determined using Scanning electron microscope (SEM, JSM 6490 LA, JEOL). The phase analysis was determined by energy dispersive spectroscopy (EDS, JSM 6490 LA, JEOL).

2.2.4. Surface profilometry

A scan length of 10 mm with a 1 µm spacing at the measurement speed of 50 µm s⁻¹ was used to assess average roughness using a surface profilometer (VECO DEKTAK 150). Five specimens from each group (coated and uncoated wires immersed for a period of 0, 1, 10 and 30 days) were utilized for line scans and the mean roughness parameters were obtained.

2.2.5. Nanoindentation test

The mechanical performance of sputtered titanium coatings on Ni–Ti arch wires were determined using nanoindentation test. Nanoindentation of both coated and uncoated wires immersed in artificial saliva for 0, 1, 10 and 30 days were carried out to determine the variation in hardness and elastic modulus with respect to time period. For each given condition 15 trials were performed. All nanoindentation testing (Triboindenter, TI-950) was carried out at 25°C by using a Berkovich indenter.

2.2.6. Linear scratch test

For linear scratch test, the samples were mounted in acrylic resin and tests were performed using a micro-combi tester (MCT S/N 06-0213, CSEM, Switzerland) with a Rockwell diamond indenter with a tip radius of 100 µm, at a speed of 6 mm min⁻¹, loading rate of 9.97 N min⁻¹, load range of 0.03–15 N and scratch length of 6 mm. The equipment is interfaced with an integrated optical microscope, an acoustic emission (AE) detection system and a tangential friction force sensor.

2.2.7. Inductive coupled plasma emission spectroscopy (ICPE) analysis

Concentration of nickel ions released into the artificial saliva from the coated and uncoated Ni–Ti wires by the end of 30 days were determined using ICP analysis [ARL 3410 ICP].

3. Results

Figure 1(a) displays the cross-sectional SEM image of the titanium sputter coated NiTi arch wire. A uniform dense titanium coating layer was observed where the thickness varied from ∼3–5 µm. EDS analysis of the coating showed presence of titanium peaks only, indicating the absence of any secondary phase formation during the sputtering process (Fig. 1(b)). EDS analysis of the substrate showed the presence of both nickel and titanium peaks (Fig. 1(c)).

Fig. 1.

(a) Cross-sectional SEM image of the titanium sputter coated Ni–Ti archwire, (b) EDS analysis of the coating, and (c) EDS analysis of the substrate (Ni–Ti). (Colors are visible in the online version of the article; https://dx-doi-org.web.bisu.edu.cn/10.3233/BME-151550.)

Figure 2 shows the SEM surface images of coated (Fig. 2(a) and (b)) and uncoated samples (Fig. 2(c) and (d)) subjected to 1 and 30 days in artificial saliva. The physical appearance of the surface showed relatively smooth morphology for coated samples when compared to uncoated samples, indicating no abrasion/pitting like features due to corrosion.

Fig. 2.

(a) and (b) SEM surface images of coated samples subjected to 1 and 30 days in artificial saliva and (c) and (d) SEM surface images of uncoated samples subjected to 1 and 30 days in artificial saliva.

Statistically significant percentage difference in the surface roughness values for the coated and uncoated samples with respect to the time period were observed ( $p < 0.05$ ). By the end of 30 days, uncoated samples showed 44.40 times increase in the surface roughness values. The coated samples showed only 6.88 times increase in the surface roughness values (Table 1). Submicronic surface profiling indicated that there was an increment increase in the surface roughness in uncoated samples with increase in the immersion time (Fig. 3). The coated samples showed relatively no change in roughness values.

The hardness and elastic modulus values of the uncoated samples for a given time period were statistically evaluated using paired t test. Statistical evaluation revealed significant difference ( $p < 0.05$ ) in the hardness and elastic modulus values between 0, 1, 10 and 30 days for the uncoated samples (Table 2). Uncoated samples exhibited a decrement trend in hardness and elastic modulus.

The hardness and elastic modulus values of the coated samples for a given time period were statistically evaluated using paired t test. There was no statistical difference in the elastic modulus values between 0, 1, 10 and 30 days for the coated samples ( $p > 0.05$ ) (Table 2). The coated samples exhibited minimal variations in the hardness and elastic modulus values.

The coatings were found to be relatively stable on linear scratch test. Figure 4(a) shows the critical load (Lc) at which a sharp peak in the AE curve is observed for the titanium sputter coated NiTi arch wires with reduced frictional coefficient compared to uncoated samples (Fig. 4(b)). The mean Lc value of the titanium coated NiTi arch wire was 8.53 ± 0.22.

Optical imaging of these scratched surfaces showed (Fig. 5(a)) plastic deformation initially and longitudinal cracks at the borders of the scratch track. Extended scratch tracks (Fig. 5(b), (c)) showed spallation along the track border, indicating these coatings were both cohesive and interfacially adherent to the substrate. This indicates that these coatings have the capability to withstand flexural stresses as well as extended clinical use in the oral cavity.

ICPE tests of post immersed artificial saliva solution of uncoated samples indicated statistically significant difference (

p < 0.05

) in the amount of nickel and titanium (Fig. 6) release within all time periods (from 1 to 30 days). Titanium sputtered coated NiTi wires indeed was effective in inhibiting nickel release. No titanium and/or nickel ions were detected in the artificial saliva solution.

Table 1

Percentage difference in the surface roughness values for the coated and uncoated samples with respect to the time period

Time period	Uncoated surface roughness values, mean ± SD	Coated surface roughness values, mean ± SD	p value
Difference from 0–1 days (%)	−13.88 ± 4.66	−6.43 ± 1.99	0.000
Difference from 0–10 days (%)	−26.08 ± 4.01	−4.39 ± 3.70	0.000
Difference from 0–30 days (%)	−44.40 ± 7.26	−6.88 ± 3.14	0.000

Fig. 3.

Variations in surface roughness with increase in immersion time. (Colors are visible in the online version of the article; https://dx-doi-org.web.bisu.edu.cn/10.3233/BME-151550.)

4. Discussion

In vivo studies on the use of NiTi orthodontic devices have reported several cases of severe inflammatory reactions resulting in contact dermatitis [23,24] One of the mechanisms that is associated with the nickel release from NiTi alloy is the intergranular corrosion (Fig. 7) occurring due to the galvanic coupling of two phases. The coating and surface modification of NiTi archwires are considered a favorable option for improving its corrosion resistance and biocompatibility.

Table 2
Hardness and elastic modulus of coated and uncoated samples obtained from nanoindentation

Days	Coated wires		Uncoated wires

	Hardness (GPa)	Elastic modulus (GPa)	Hardness (GPa)	Elastic modulus (GPa)
0	1.32 ± 0.07	55.58 ± 7.64	6.94 ± 0.15	79.77 ± 2.71
1	1.24 ± 0.10	54.28 ± 7.24	6.69 ± 0.12	74.24 ± 4.55
10	1.37 ± 0.10	54.46 ± 5.09	6.48 ± 0.12	67.96 ± 5.32
30	1.30 ± 0.09	56.76 ± 4.39	6.19 ± 0.05	58.16 ± 6.02

Fig. 4.

(a) Acoustic emission graph obtained through a linear scratch test and (b) frictional coefficient of uncoated samples compared with coated samples. (Colors are visible in the online version of the article; https://dx-doi-org.web.bisu.edu.cn/10.3233/BME-151550.)

Fig. 5.

(a)–(c) Optical images of the scratched surfaces. (Colors are visible in the online version of the article; https://dx-doi-org.web.bisu.edu.cn/10.3233/BME-151550.)

Fig. 6.

Quantitative graph indicating the nickel and titanium detected in the artificial saliva.

Fig. 7.

Schematic representation of the inter-grannular corrosion occurring in the Ni–Ti alloy. (Colors are visible in the online version of the article; https://dx-doi-org.web.bisu.edu.cn/10.3233/BME-151550.)

Most of the available coatings were not effective in reducing the nickel release, and were affecting the mechanical properties of the NiTi arch wires because of the high temperature involved in the coating process [14–17]. So in this study, high vacuum plasma sputtering technique, which is a low temperature coating process was used to form a protective layer onto NiTi substrate. High vacuum plasma sputtering by forming titanium based bonds at molecular level reduces the nickel release without deteriorating the superelasticity of NiTi wires. The heating process of approximately 50–80°C was not found to affect the mechanical properties of the NiTi.

Submicronic surface profiling indicated that there was an incremental increase in the surface roughness in uncoated samples with increase in the immersion time. The coated samples showed relatively no change in roughness values with increase in the immersion time. According to Robert et al. [25] increased surface roughness can increase frictional forces because it enhances the contact area between the bracket and the wire. The coated samples showed only 6.88 times increase in the surface roughness, which indicates that these coatings could be beneficial where high friction is an issue in orthodontics.

To evaluate the mechanical properties of titanium coating on exposure to artificial saliva, nanoindentation tests were performed. It can be challenging to determine the true mechanical properties of a material that exists only as a thin coating, less than a few micrometers thick. The recent development of the nanoindentation technique, which offers nanometer-scale resolution, has allowed such measurements to be successfully performed. The nanoindentation test is convenient for orthodontic alloys, because it requires only a small volume of specimen material [26]. This test is a suitable method for the evaluation of near-surface mechanical properties of materials. The loading and unloading behavior of both coated and uncoated samples showed similar trend but with variability in peak load values (Fig. 8). This was obvious since Ni–Ti is a harder alloy than titanium metal. The advantage of these indentations comes from the fact that variation in localized areas can be detected which can project the reliability performance of the coating/substrate under consideration. Variation in mechanical properties across the surface is an indication of the vulnerability of uncoated samples to mechanical loading. The decreasing hardness and elastic modulus of the uncoated sample in the artificial saliva indicates that it exhibit little mechanical support at the surface zone when subjected to a corrosive environment.

Fig. 8.

Typical loading-unloading curves for coated and uncoated samples obtained by the nanoindentation test. (Colors are visible in the online version of the article; https://dx-doi-org.web.bisu.edu.cn/10.3233/BME-151550.)

Sputtering of titanium ensures that fine particles (∼300 nm) form a dense layer onto Ni–Ti substrate. Samsonov et al. [27] reported that corrosion behaviour of titanium is greatly affected by its grain size. By reducing the average grain size to the ultrafine crystalline range they seem to exhibit enhanced corrosion resistance. This was reflected as similar hardness and elastic modulus values in coated samples over a given time period indicating that the coating retained its structural stability in artificial saliva.

Thin film coatings with plasma sputtering are advantageous in orthodontics as they offer increased wear and corrosion resistance, reduced interfacial friction and prolonged substrate life [18]. The coatings were found to be relatively stable on linear scratch test with reduced frictional coefficient compared to uncoated samples.

Mechanical performance and efficiency of titanium sputter coated NiTi arch wires in inhibiting nickel release seems to be satisfactory under in vitro conditions. Preliminary data shows that nickel release could be reduced by using titanium sputter coated NiTi arch wires. By the end of 30 days, nickel ion concentration in the artificial saliva from the uncoated samples were around 0.412 ppm (412 ppb). Study by Liu et al. [28] have shown that stressed NiTi wires exhibited substantial increase in the nickel release compared with the unstressed specimens. So in the oral environment the nickel release from the uncoated samples will be much higher than those obtained in the present study. High vacuum plasma sputter coating can be validated for its performance after performing in vivo tests. The effects of pH variations of the saliva on the structural integrity of the titanium sputtered NiTi arch wires need to be studied. This preliminary study was performed under simulated conditions. The effects of masticatory loads and shear forces produced during brushing and archwire bracket interactions needs to be investigated.

5. Conclusions

The present study upon evaluating the stability, mechanical performance and prevention of nickel release of titanium sputter coated NiTi archwires, revealed the following.

High vacuum plasma ion titanium sputter coatings seem to be promising for nickel sensitive patients.

The coatings obtained are uniform and show minimal hardness and elastic modulus variations in artificial saliva over a given time period.

SEM analysis showed no peeling/exfoliation of the coating, indicating good adherence property of these coatings.

Submicronic surface profiling of the coated samples indicated the roughness values of coated samples were ∼50% lower than their uncoated counterparts indicating that these coatings could be beneficial during the orthodontic tooth movement.

Titanium coatings exhibited high reliability on exposure even for a prolonged period of 30 days in artificial saliva.

The coatings were found to be relatively stable on linear scratch test (>8 N to produce an initial disturbance in acoustic emission tests).

Titanium sputtering seem to adhere well with the Ni–Ti substrates at the molecular level, this was further confirmed by ICPE analysis which showed no dissolution of nickel in the artificial saliva.

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